This invention relates to a positive displacement device. More particularly, this invention relates to a positive displacement pump or metering device which is capable of dispensing fluids from a source or reservoir at extremely high accuracy by providing seals which prevent the flow of fluid from the dispensing chamber other than in the desired direction for taking the fluid in and discharging same.
Shaft seals for reciprocating pistons and rods which move in a sealed chamber have commonly included a rubber O-ring. Often the O-ring is used in combination with one or more backup rings. The rubber O-ring tangentially contacts the sliding shaft or rod and it has been found that in use the O-ring tends to twist and under conditions of stress the O-ring will actually twist and become extruded along the reciprocating shaft or rod or out over the backup rings. Obviously, this leads to rapid breakdown in the seal and unwanted leakage of the fluid being handled. The seal arrangements of this type are shown in U.S. Pat. Nos. 2, 973,978, issued Mar. 7, 1961 and 3,171,334, issued Mar. 2, 1965.
The disadvantages encountered with O-ring seals can be avoided to a certain extent by using a bearing material such as Teflon which is capable of allowing sliding contact under pressure. Such a device is disclosed in U.S. Pat. No. 2,905,489, issued Sept. 2, 1959. In this arrangement, however, rubber-like sealing rings are in contact with the reciprocating or turning shaft and again twisting and unwanted extrusion can occur leading to breakdown in the seal. In this arrangement a U-shaped member facing in the longitudinal direction of the shaft receives a flexible sealing ring which is urged into the U-shaped channel ring in response to the back pressure. This variable pressure loading causes continual flexing and unflexing of the rubber-like sealing ring and the relative movement between the sealing ring and the channel member causes eventual wear and failure of the seal.
Annular shaft or rod seals are extremely critical in devices for dispensing or metering fluids particularly liquids with a high degree of accuracy.
Obviously, if there is any leakage of the fluid being dispensed or metered through the seal, then the accuracy of the device is severly impaired. Because of the lack of completely efficient seals, dispensing and metering devices with a high degree of accuracy are not available.
The present invention, therefore, provides a positive dispensing displacement device with an extremely high degree of accuracy and can thus be used in chemical, analytical, pharmaceutical and medical applications where extremely fine accuracy is required.